In a typical internal combustion engine, including a piston and ring assembly reciprocable within an associated cylinder bore, the majority of the cylinder wall wear occurs at the upper portion of the cylinder bore. This is the area of the bore where the face of the one or more piston rings frictionally engages the bore with a scraping action against the cylinder bore surface. In contrast, the lower end of the cylinder bore wall is more lightly loaded, with the piston skirt causing measurably less wear in this lower wall area. As a consequence of these discrepancies in cylinder wear, a cylinder bore tends to become gradually tapered, i.e., exhibiting a relatively larger diameter at the top than at the bottom.
The bore of the cylinder also exhibits considerably more wear in a direction "across" the engine, that is, at those portions oriented 90 degrees to the piston pin, than in a direction along the length of the engine (i.e., in alignment with the piston pin). This phenomenon results from the significantly higher loads exerted by the piston in the direction across the engine as the piston reciprocates within the cylinder bore due to the angularity of the connecting rod with respect to the piston pin. During the power stroke of the engine, the total force pushing down on the piston (due to combustion gas pressure) may often be of a magnitude of many tons of pressure. This extreme force acts against the piston to jam the piston with a side load against the cylinder wall. There is relatively little side loading in the lengthwise direction of the engine (parallel to the piston pins and crank shaft journals) because the connecting rod is straight (i.e., non-angular) at all times with respect to those portions of the cylinder bore. Additional side loads are created by inertia forces of the piston, which forces increase significantly with higher piston weights.
The above-described piston side loads result in the cylinder bore exhibiting wear in an oval shape. Since the heaviest side loads occur during the power stroke, the side of the bore which is loaded during this period of the four-stroke cycle exhibits the most wear. This portion of the cylinder bore is normally referred to as the major thrust side of the bore, with the opposite upper surface of the bore being referred to as the minor thrust side. In the majority of engines currently built and which rotate counterclockwise (as viewed from the rear), the major thrust side is located at the right side of the bore (when viewed from the rear).
In addition to the two above-described normal types of wear (which simultaneously cause the cylinder bore to become tapered, as well as out-of-round), the cylinder bore will often deviate from a true cylinder because of strains caused by unevenly torqued cylinder head fasteners. Distortion can also be caused by abnormal engine temperatures due to general overheating of the engine cooling system, or localized overheating caused by restrictive or clogged cooling passages. These uncontrolled heat effects may cause "low" and "high" spots in the cylinder bore, and may result in the bore wearing to a "wavy" surface (along the axis of the bore) instead of a relatively even taper.
The one or more piston rings of a piston and ring assembly should ideally exert sufficient pressure against the cylinder bore to form a tight seal, thereby preventing leakage of combustion gasses downwardly, and preventing movement of oil upwardly. When a piston ring exerts more pressure than is required to create an effective seal, the result is an undesirable increase in piston ring and cylinder wall wear, and increased engine friction which reduces power, increases engine heat, and raises fuel consumption.
The sides of the piston rings (i.e., the top and bottom surfaces), and the piston ring lands of the piston (which contain the rings) also exhibit wear. While the pistons of an engine move the rings upwardly and downwardly with respect to the cylinder walls, the rings are in constant sideways motion (radially of the piston) to accommodate their reaction to irregularities on the surface of the cylinder wall, and to accommodate movement of the pistons due to side loads. When the top of the piston moves toward the cylinder wall (from side loading) the ring will be forced back into the piston ring groove. There must be sufficient clearance available, in a radial direction behind the ring, so that the ring face may be forced inwardly to become flush with the edge of the piston, without the piston ring "bottoming-out" (in the radial direction) against the back wall of the ring groove. If the piston ring does bottom-out, the impact of the combustion and inertia forces acting upon the piston will be transmitted to the ring, and the ring will eventually break. In order to assure that bottoming-out is avoided, all piston ring lands are machined so that there is normally between 0.005 inches and 0.015 inches clearance radially behind the ring, when the ring face is flush with the outer radial surface of the piston. The space that is established behind the ring is normally referred to as the "back wall area", or the "back wall clearance".
The back wall area also functions to increase the sealing pressure of the ring face on the cylinder bore wall during the combustion stroke, when the normal top and bottom piston ring clearance (i.e., its axial clearance) is all at the top of the ring due to combustion forces pushing the ring tightly against the bottom of the ring groove. The combustion gasses pass though this axial clearance, and raise the gas pressure in the back wall area, thereby forcing the piston ring outwardly to seal more tightly against the cylinder bore wall. To enhance this effect, the back or inside surface of the top piston ring of a piston and ring assembly is typically cut with a chamfer, thereby decreasing the time required for creating sealing pressure in the back wall area, and increasing the pressure therein. When such a chamfer is made in the upper edge of the ring, the combustion gas will flow more readily into the back wall area because the sharp edge of the ring has been removed, thereby reducing turbulence and "squeeze" of the combustion gas. The ring-to-cylinder wall pressure will also be increased because the effective surface area acted upon by the combustion gas is relatively increased.
One of the problems exhibited by all current piston designs is that when the ring bounces, or flutters, within the cylinder bore, the seal at the ring face to the cylinder wall is momentarily lost, and combustion gas leaks past the ring face. This results in a drop in pressure in the back wall area, further reducing the ability of the ring to seal tightly against the cylinder bore wall. Such ring bounce is most often caused by irregularities on the cylinder wall surface (i.e., such as "waviness" described above) or by rapid shifts in the piston from the major thrust side to the minor thrust side of the cylinder bore. Both of these phenomenons occur at higher engine (and piston) speeds. Ring flutter is usually caused by combustion pre-detonation or pre-ignition, which can cause high speed shock waves in the cylinder, and which vibrate the ring causing it to lift off of the cylinder wall.
On the compression stroke of the engine, the compression (i.e., intake charge) pressure pushes down on the piston while the connecting rod resists this pressure by its connection to the piston pin. The combined action of these two forces, in all reciprocating piston engines, pushes or thrusts the piston against that side of the cylinder bore toward which the connecting rod is angled from its connection to the associated crankshaft.
In contrast, during the power stroke, the connecting rod slopes angularly toward the opposite side of the cylinder bore. Combustion gas pushes downward on the piston, and the connecting rod resists this pressure by pushing upward on the piston pin. The combination of these two forces pushes or thrusts the piston against that surface of the cylinder bore opposite the side against which it is pushed during the compression stroke.
In the majority of engines, previously described, the direction of the side thrust acting on the piston changes from one side to the other (from left to right when viewed from the rear) as the piston moves through top dead center (TDC). Within the period from 60 to 0 degrees before top dead center, the piston is thrust (by compression) to the left side of the cylinder bore, with transferring of the side thrust thereafter to the opposite, right side, within about 0 to 10 degrees after the piston passes through top dead center. This change in direction of thrust pulls the piston away from the left side of the bore, and "slaps" it against the right side. If the clearance between the piston and the bore is excessive, an audible noise is heard which is referred to as "piston slap". Excessive clearance can be intentionally provided, such as in racing engines where extra piston clearance is provided because of high piston metal operating temperatures. Excessive clearance may also result from cylinder bore wear described above.
In current engines, which include aluminum pistons, there will ordinarily be no audible piston slap when the pistons and cylinder bores have not been subjected to wear. However, there is ordinarily some degree of thrust rocking occurring. Even if there is audible piston slap (due to inaccurate machining) during the period in which the engine is warming-up, the aluminum pistons usually heat quickly, and expand, thereby reducing the piston/cylinder clearance and eliminating the slapping noise. However, in some instances, current engines are required to operate by design (machine tolerances or load requirements) with excessive piston clearance, and low levels of audible piston slap can exist at all operating conditions.
Rather than being manufactured perfectly round, modern pistons are ground slightly oval ("cam ground"), with the piston typically having a diameter across the pin hole which is about 0.009 inches less than that diameter perpendicular to the hole. Usually, current aluminum pistons are manufactured such that the skirts are ground about 0.0005 inches larger in diameter at the bottom of the skirt. In other words, the skirt flares or tapers outwardly by about this dimension.
After extended periods of service, the thrust forces acting upon the piston skirt gradually reduce its diameter so that the skirt then tapers inwardly, instead of outwardly as described above (i.e., the skirt "collapses"). This reduction in skirt diameter is a result of impacts on the piston skirt caused by the thrusting action of the piston, and is in addition to any normal surface wear of the skirt resulting from friction. Skirt collapse increases the clearance between the cylinder bore and the piston skirt, and results in increased piston slap.
Piston slap can be envisioned as a rocking motion of the piston in the cylinder bore. The rocking action of the piston directly affects the ability of the piston rings to seal, thereby reducing their effectiveness. First, as the piston rocks when new, the unworn piston ring, with a flat surface against the wall, will also be rocked with the piston. The rocking action of the ring face will alternately move the seal area of the ring from the uppermost edge to the lowermost edge of the ring, instead of using the entire ring face. The stresses placed upon these outer ring edges, by the rocking of the piston, rounds off the outer faces of the rings, and further reduces their effectiveness. As the piston rocks left, the lower ring edge is worn away, and as the piston rocks right, the upper edge is worn away. Gradually, as wear due to thrust-rocking continues, the entire ring face is rounded so that even when the piston is vertical in the bore, only a small tangent of the ring face is available to seal the cylinder. Gas pressure leaks down past these rounded surfaces, and oil leaks upward into the combustion chamber affecting emissions and consuming oil.
In recent years, there have been attempts to reduce leakage of combustion gasses past the rings into the crankcase of the engine. Such attempts have been made in order to increase the peak power of the engine, and the specific power of the engine in relation to the fuel consumed (referred to as brake specific fuel consumption of the engine, BSFC). One such method used during recent years is to slightly angle grind the face of the ring which contacts the cylinder wall. This is termed a "tapered face" piston ring and it is designed to establish a single contact point (when viewed in cross-section) at the top of the ring, during early operation of the new engine, which then moves down across the face of the ring from progressive wear. The intent is that the ring will reduce bounce when shifting contact points from the top edge to bottom edge as the piston rocks in the cylinder bore (i.e., the most tapered (lowest) edge will not contact the wall as severely as the least tapered (highest) edge). In some instances where ring flexing is addressed, the angle grind may be opposite to the foregoing (i.e., contact point at the bottom). However, the intent of a single point contact and progressive wear across the ring, from top to bottom, renders the same result. To date, this approach has had some minor improvements realized, but has not significantly corrected the problems. Additionally, attempts have been directed at sealing the end gaps of the rings, which may normally range from a clearance of 0.008 inches up to 0.030 inches per ring. Such gap sealing constructions, which normally use two or more interworking rings, are sometimes referred to as "gapless" or "zero gap". Finally, some attempts have been noted of the use of metallic and non-metallic gas seals on the back wall side of the piston ring whereby it is intended to seal, or entrap combustion gasses, attempting to pass around the back side of the piston groove. However, heretofore none of these attempts of back wall sealing are known to the present inventor to have addressed the movement of the piston, within the bore, or the need to support the ring with a compressible member which limits the rings ability to move radially into the piston groove. These attempts have not recognized or addressed the problems during dynamic motion of the piston caused by piston thrust, rocking, and the required back wall clearance. In addition, these previous attempts have not corrected the losses in engine efficiency which occur during conditions which cause the piston ring to flutter, bounce, and erode away the sealing surface. In fact, all such previous attempts known to the present inventor specifically address the need to allow the piston ring to move freely within the ring groove radially to the full extent in order to avoid bottoming (loading) the ring between the piston groove back wall and the cylinder bore wall when the piston rocks, or thrusts, toward the wall.